In general, a wind turbine comprises a hub to which the rotor blades are mounted. The hub is mounted to a rotatable structure of the wind turbine. The rotatable structure rotates around a stator of the wind turbine, wherein the stator is mounted inside a nacelle of the wind turbine. In some embodiments of wind turbines, gear devices are installed to transfer the drive torque induced by the hub to a generator rotor inside a nacelle of the wind turbine.
In other wind turbine embodiments, the so-called direct drive wind turbine design is applied. In a direct drive wind turbine design, no gear devices are used. The drive torque is directly transferred from the hub to the rotatable structure which is directly coupled to a generator rotor of the wind turbine. Hence, in the direct drive wind turbine design, large diameters of the rotatable structure and respective stator parts of stationary structures of the wind turbine, in particular inside the generator, exist.
When a generator rotor of a large direct drive wind turbine is rotating and heavy shifting loads occur, e.g. due to wind gusts, large turbulences and/or if a bearing of the generator of the wind turbine is damaged, then it is important to ensure that there is kept at least a small air gap between the rotatable structure and the stationary structure of the generator. Otherwise, there is a risk that e.g. the generator may be damaged.